Method of producing photoelectric conversion device

ABSTRACT

The present invention is directed to a method of manufacturing a photovoltaic cell with high conversion efficiency, wherein a polycrystal CdTe layer with a large grain size can be formed by forming an indium oxide film (20) on a transparent conductive substrate having a transparent conductive film (2) as its surface layer, then forming an n-type CdS layer (3) and a p-type CdTe layer (4) thereon, then attaching cadmium chloride (CdCl 2 ) on the p-type CdTe layer, and then annealing. The indium oxide film (20) is capable of relaxing strain caused at an interface between the transparent conductive film (2) and the n-type CdS layer (3), so that a good CdS/CdTe junction interface can be formed. The indium oxide film (20) can be formed by forming an indium film on the transparent conductive substrate and then annealing in oxygen containing atmosphere.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photovoltaic cell for convertingsolarlight energy into electrical energy and, more particularly, amanufacturing method to enhance conversion efficiency of a photoelectricconversion device such as a thin film photovoltaic cell, which employs aCdTe thin film formed on a transparent substrate such as a glasssubstrate as a light absorber layer.

2. Description of the Prior Art

It has been well known that CdTe has a high absorption coefficient ofmore than 10⁴ cm⁻¹ and its thin film of about 5 μm thickness is capableof sufficiently absorbing the solarlight. The CdTe thin film ispromising as material for the thin film photovoltaic cell since it iseasy to form a polycrystal film of high quality by virtue of variousthin film forming methods such as a printing method, a plating method,an evaporation method, etc. In addition, a band gap of CdTe (˜1.47 eV)is most suitable for solarlight spectrum amongst various materials forthe photovoltaic cell, and thus the highest conversion efficiency can beexpected. It has also been calculated that theoretical efficiency of aCdTe thin film photovoltaic cell is in excess of 20%. However, thehighest value of the conversion efficiency of the CdTe thin filmphotovoltaic cell which has been reported up to now is about 15%, whichis largely different from a theoretical value. Like the above, thephotovoltaic cell employing the CdTe thin film as the light absorberlayer has been expected as the low cost and high efficiency photovoltaiccell, but it is difficult under the existing circumstances tomanufacture the photovoltaic cell which employs the CdTe thin filmhaving sufficiently high conversion efficiency as the light absorberlayer with good reproducibility.

As the photovoltaic cell employing the CdTe thin film as the lightabsorber layer in the prior art, a pn junction photovoltaic cell iscommon which is formed by depositing a p-type CdTe layer 4 on an n-typeCdS layer 3, as shown in FIG. 1. Though illustrated upside down in FIG.1, a transparent conductive film 2 such as an indium-tin oxide film (ITOfilm) is formed on a glass substrate 1, then the n-type CdS layer 3 isformed 0.1 to 10 μm thick thereon, then the p-type CdTe layer 4 isformed 1 to 10 μm thick thereon, and then an ohmic electrode 5 made ofCu/Au, etc. is formed thereon. If a sheet resistance of the CdS layer 3is sufficiently small, the transparent conductive film 2 may be omitted.

In the prior art, the CdTe/CdS photovoltaic cell shown in FIG. 1 hasbeen manufactured according to manufacturing steps described in thefollowing. That is, the cell has been manufactured according tofollowing steps (1) to (5) (this is called "a first manufacturing methodin the prior art" hereinafter):

(1) The transparent conductive film 2 such as the indium-tin oxide (ITO)is deposited by the sputtering method, etc. on the glass substrate 1such as a Corning 7059 substrate to have a thickness of about 150 nm to1 μm such that a sheet resistance of less than 10 Ω/□ can be given.

(2) The n-type CdS layer 3 is deposited by the vacuum evaporationmethod, etc. at a substrate temperature 350° C. to have a thickness of0.1 to 10 μm. If a sheet resistance of the CdS layer 3 is enoughlysmall, there is no necessity of the above transparent conductive film 2.

(3) Next, either CdTe molecules or Cd and Te with corresponding molefraction are deposited 1 to 10 μm thick by means of a screen printingmethod, an electrolytic plating method, a spray method, or the like.

(4) Cadmium chloride (CdCl₂) or chlorine (Cl₂) is then mixed with oradded to such CdTe molecules or Cd and Te with corresponding molefraction in proper quantity. In turn, a resultant structure is annealedin an air or an inert gas at a temperature of 350 to 700° C. for about0.1 to 2 hours to thus obtain the p-type CdTe layer 4 which has asubstantially equal stoichiometry.

(5) Finally, the ohmic electrode 5 is formed on the p-type CdTe layer 4.For instances a composite film made up of a 10 nm thick Cu layer and a100 nm thick Au layer may be used as the ohmic electrode 5.

The CdTe/CdS photovoltaic cell shown in FIG. 1 can also be manufacturedaccording to following manufacturing steps (this is called "a secondmanufacturing method in the prior art" hereinafter). That is, accordingto the second manufacturing method in the prior art, the CdTe/CdSphotovoltaic cell has been manufactured based on following steps (1) to(5):

(1) The transparent conductive film 2 such as the ITO film is depositedabout 200 nm thick by the sputtering method, etc. on the glass substrate1 such as the Corning 7059 substrate.

(2) The n-type CdS film layer is deposited by the vacuum evaporationmethods etc. at a substrate temperature 350° C. to have a thickness of0.1 to 10 μm, e.g., a thickness of 0.15 μM.

(3) A p-type CdTe layer is formed by the vacuum evaporation method at asubstrate temperature 350° C. to have a thickness of about 4 μm.

(4) The p-type CdTe layer is dipped in a methanol (CH₃ OH) solutioncontaining copper chloride (CuCl₂) or a CH₃ OH solution containing CuCl₂and CdCl₂, then is dried by natural drying, and then is annealed at 400°C. for 15 minutes in an N₂ +O₂ (4:1) atmosphere.

(5) A surface of the CdTe layer is etched by using a K₂ Cr₂ O₇ +H₂ SO₄+H₂ O solution, etc., then Cu (10 nm)/Au (100 nm) are deposited by thevacuum evaporation, and then annealed at 150 ° C. for three hours in theair, whereby resulting in the ohmic electrode 5.

In the prior art, it has been appreciated that the cadmium chloride(CdCl₂), the copper chloride (CuCl₂), or the chlorine (Cl₂) which isused in the step (4) in the above first and second manufacturing methodscan provide such advantages that the CdS layer 3 and the CdTe layer 4serving as polycrystal films are grown in grain size several times toseveral tens times larger than before, conductivity of the CdTe layer 4is changed into p-type conductivity so as to form a pn junction betweenthe n-type CdS and the p-type CdTe, and interfacial diffusion throughthe CdS/CdTe junction is facilitated to form a gradient compositionlayer so that generation of the defects due to the lattice mismatchingcan be prevented. In this manner, the case is popular in the methods inthe prior art where either chloride such as cadmium chloride or chlorineis introduced during deposition of either the CdTe molecules or Cd andTe with corresponding mole fraction. In this case, in order to enhancephotoelectric conversion efficiency of the CdTe photovoltaic cell, thechloride or chlorine has to be controlled to its optimum amountaccording to respective different deposition methods. The optimum amountmust not be too much reduced nor increased, and it must be controlled toproper quantity.

In the above method of manufacturing the CdTe/CdS photovoltaic cell inthe prior art, there has been such a problem that, when the CdS layer 3and the CdTe layer 4 are grown in grain size approximately ten times inthe above step (4), strain is caused at an interface between thetransparent conductive film 2 and the CdS layer 3 and subsequently thisstrain causes another strain at the CdS/CdTe junction interface, therebydecreasing an open-circuit voltage Voc and a fill factor FF of the CdTephotovoltaic cell. Hence it is difficult to manufacture the CdTe/CdSphotovoltaic cell having sufficiently high conversion efficiency withgood reproducibility.

Although, as will be described in detail later, the present inventionincludes formation of a thin film such as an indium oxide film (In₂ O₃film), a tin oxide film (SnO₂ film), or the like between a conductivesubstrate and an n-type semiconductor layer as one of features,techniques have been set forth in Patent Application Publications(KOHYOs) 8-500209 and 8-500210 wherein a high conductivity conductivelayer such as tin oxide and a low conductivity conductive layer areformed on the glass substrate and then an n-type semiconductor layer(n-type CdS layer) is formed thereon. However, in order to improvequantum efficiency with respect to short wavelength light, thesetechniques have been employed to make the CdS layer extremely thin andtherefore such techniques substantially differ from the presentinvention. In other words, the low conductivity conductive layers whichhave been set forth in Patent Application Publications (KOHYOs) 8-500209and 8-500210 are associated with a technique to avoid a disadvantagecaused by pinholes, etc. generated in the CdS layer when the CdS layeris made extremely thin less than 50 nm. More particularly, the lowconductivity conductive layers are needed as an n-type backupheterojunction material to avoid short circuit of the photovoltaic celldue to the pinholes, etc. in the CdS layer whereas the high conductivityconductive layers are respectively such layers that are necessary tomake ohmic contact between the low conductivity conductive layer and theelectrode layer of the photovoltaic cell. In these documents, anelectric power is in fact generated between the low conductivityconductive layer and the p-type CdTe layer and thus there aredescriptions to the effect that the CdS layer may be removed completely.Accordingly, it can be understood from these descriptions that thetechniques set forth in the above Patent Application Publication (KOHYO)8-500209, etc. are different techniques from the present invention,which have objects, functions, and effectivenesses being different fromthose of the present invention.

SUMMARY OF THE INVENTION

The present invention has been made to remove the above drawbacks, andit is an object of the present invention to provide a manufacturingmethod which is able to provide a photoelectric conversion device withsufficiently high conversion efficiency.

In order to achieve the above object, as shown in FIGS. 2A to 2G, themethod of manufacturing the photoelectric conversion device according tothe present invention is characterized by comprising at least,

(a) a first step of forming an indium oxide film (In₂ O₃ film), a tinoxide film (SnO₂ film), or an indium-tin film (ITO film) 20 by forming afilm made of an indium film (In film), a tin film (Sn film), or anindium-tin alloy film (In-Sn alloy film) on a transparent conductivesubstrate and then by annealing the film at an oxidizing temperature(usually, 300 to 500° C.) for oxidizing time (usually, within one hour)in oxygen containing atmosphere (FIG. 2C).

(b) a second step of forming an n-type semiconductor layer 3 on the In₂O₃ film, the SnO₂ film, or the ITO film 20 and then forming a p-typesemiconductor layer 4 on the n-type semiconductor layer 3 (FIG. 2D, FIG.2E), and

(c) a third step of enlarging grain size of polycrystals of the p-typesemiconductor layer 4 (FIG. 2F). In this event, it is preferable thatthe transparent conductive substrate is formed by forming a transparentconductive film 2 on a transparent substrate 1 such as the glasssubstrate. The ITO film, zinc oxide (ZnO) film, or SnO₂ film may be usedas the transparent conductive film 2. In addition, CdS, CdZnS, ZnS, orthe like is desirable for the n-type semiconductor layer 3 and alsoCdTe, CuInSe₂, CuInGaSeS, or the like is desirable for the p-typesemiconductor layer 4. If CdTe is employed as the p-type semiconductorlayer 4, the step of enlarging grain size of polycrystals in the thirdstep may be carried out by forming a chloride layer 41 on the p-typesemiconductor layer 4 and then annealing, otherwise annealing under achlorine vapor pressure. If CuInSe is employed as the p-typesemiconductor layer 4, the step of enlarging grain size of polycrystalsmay be carried out by forming a layer 41 made of selenium, selenide,sulfur, or sulfide on the p-type semiconductor layer 4 and thenannealing, otherwise annealing under a selenium or sulfur vaporpressure. Although cadmium chloride (CdCl₂) is desirable for thechloride 41, other chlorides may be employed and likewise chlorine (Cl₂)itself may be employed.

In order to adsorb CdCl₂ on the p-type semiconductor layer 4, after amethanol solution containing CdCl₂ is coated on the p-type semiconductorlayer 4, the layer 4 is dried by virtue of natural drying or else warmedin a sealed vessel which can shut off the ambient air. In particular, inorder to attach a large amount of CdCl₂ in excess of 0.1 mg/cm², themethod of warming the layer 4 in the sealed vessel is preferable. Avessel such as schale which has a slightly lower sealing degree may beemployed as the sealed vessel, or another vessel which is able to sealmore perfectly may be employed. A large amount of CdCl₂ can be attachedto the surface of the p-type semiconductor layer 4 with good inplaneuniformity and in a short time by drying the methanol containing CdCl₂in the sealed vessel.

Preferably, a thickness of the In film, the Sn film, or the In-Sn alloyfilm in the first step shown in FIG. 2C should be selected in the rangeof 2.5 nm to 100 nm, especially 2.5 nm to 50 nm, and also the In film,the Sn film, or the In-Sn alloy film should be formed by the vacuumevaporation method. An oxide film, which is formed by annealing the Infilm, the Sn film, or the In-Sn alloy film in oxygen containingatmosphere, has a surface in which a grain structure having uniformgrain sizes can be achieved. Such surface is different from the surfacewhich is obtained when the oxide film is formed by the sputteringmethod, etc. and has non-uniform grain sizes and squarish grain shapetherein. This structure seems to contribute to advantages of the presentinvention. If the thickness of the In film, the Sn film, or the In-Snalloy film is set to less than 100 nm, the surface which includes thegrain structure having highly uniform grain sizes can be achieved. Inaddition, the thickness of the In film, the Sn film, or the In-Sn alloyfilm in excess of 2.5 nm is preferable since a short-circuit current Iscis increased along with the thickness if the thickness exceeds 2.5 nm.Conversely, since the open-circuit voltage Voc is decreased if thethickness is made thick, it is preferable that the thickness is set toless than 50 nm. It is also preferable that the thickness of the Infilm, the Sn film, or the In-Sn alloy film is selected not to cause theproblem of optical absorption.

In the present invention, because the In₂ O₃ film, the SnO₂ film, or theITO film 20 is formed on the transparent conductive film 2 in the firststep shown in FIG. 2C, the pn junction interface between the n-typesemiconductor layer 3 and the p-type semiconductor layer 4 can beprevented from being distorted when the n-type semiconductor layer 3such as the CdS film and the p-type semiconductor layer 4 such as theCdTe film are grown in grain size in the third step shown in FIG. 2F.This is because the In₂ O₃ film, the SnO₂ film, or the ITO film 20 mayoperate as a buffer layer between the transparent conductive film andthe n-type semiconductor layer 3 to relax strain between them.Consequently, the pn junction made up of good heterojunction such asCdS/CdTe junction can be obtained, so that crystal defects to trapcarriers at a low electric field can be reduced. As a result, the highperformance photoelectric conversion device such as CdTe/CdS can beaccomplished. In more detail, because the In₂ O₃ film, the SnO₂ film, orthe ITO film 20 is formed between the conductive substrate and then-type semiconductor layer, a reverse saturation current of the diodecan be reduced and the voltage Voc is increased. In addition, a diodefactor upon irradiating the light, i.e., an n value can be reduced whilethe fill factor FF can be improved.

As described above, the techniques disclosed in Patent ApplicationPublications (KOHYOs) 8-500209 and 8-500210 are different techniquesfrom the present invention to achieve different objects from that of thepresent invention. However, according to the present invention, thephotoelectric conversion device with considerably high efficiency andhigh uniformity rather than the photovoltaic cell set forth in thesepublications can be achieved.

A conductivity of the In₂ O₃ film, the SnO₂ film, or the ITO film 20 maybe 0.1 Ω/cm² when its thickness is about 10 nm. In other words,resistivity of the In₂ O₃ film, the SnO₂ film, or the ITO film 20 may beemployed if it is less than 105 Ω·cm. Usually such degree of resistivitycan be readily attained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a cross-sectional shape of a CdTe/CdSphotovoltaic cell in the prior art.

FIGS. 2A to 2G are views illustrating a method of manufacturing aCdTe/CdS photovoltaic cell according to a first embodiment of thepresent invention.

FIG. 3 is a plan view showing a state wherein cells are isolated on aglass substrate in the first embodiment of the present invention.

FIG. 4 is a schematic view showing a cross-sectional shape of theCdTe/CdS photovoltaic cell according to the first embodiment of thepresent invention.

FIG. 5A is a view showing a spectral-response characteristic of thephotovoltaic cell in the prior art, and FIG. 5B is a view showing aspectral-response characteristic of the CdTe/CdS photovoltaic cellaccording to the first embodiment of the present invention, whichcorresponds to this.

FIG. 6 is a view showing standard deviation in conversion efficiency ofa CdTe/CdS photovoltaic cell according to a second embodiment of thepresent invention, while comparing with that in the first embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Best modes for carrying out the invention will be explained withreference to accompanying drawings hereinafter.

(First Embodiment)

FIGS. 2A to 2G are views illustrating a method of manufacturing aCdTe/CdS photovoltaic cell according to a first embodiment of thepresent invention. In the first embodiment of the present invention,following steps are taken.

(1) After a transparent substrate 1, e.g., a Corning 7059 substrate iscleaned, as shown in FIG. 2B, an indium-tin oxide film (ITO film) 2 isthen formed 200 nm thick as a transparent conductive film by thesputtering method.

(2) Then, as shown in FIG. 2C, an indium oxide film (In₂ O₃ film) 20 isformed by depositing an indium film (In film) 10 nm thick on the aboveITO film 2 by the vacuum evaporation and then annealing at 400° C. for60 minutes in the air.

(3) Thereafter, as shown in FIG. 2D, an n-type semiconductor layer(n-type CdS layer) is formed 80 nm thick on the In₂ O₃ film 20 by thevacuum evaporation method at a substrate temperature of 150° C.

(4) Subsequently, as shown in FIG. 2E, a CdTe layer 4 serving as ap-type semiconductor layer is formed 4 μm thick by the vacuumevaporation method at a substrate temperature of 350° C . Vacuumevaporation of the CdS layer 3 and vacuum evaporation of the CdTe layer4 may be carried out successively in the same chamber. Sputtering orchemical vapor deposition method (CVD method) other than the vacuumevaporation method may be employed to form the CdTe layer. Not the CdTelayer, but Cd molecules and Te molecules with substantially equal molarnumber may be deposited on the CdS layer by the vacuum evaporationmethod, the sputtering method, or the CVD method (for purposes ofillustration, the case where the CdTe layer is deposited will beexplained hereinafter).

(5) Next, the glass substrate 1 on which the above CdTe layer/CdS layeris deposited is dipped into a methanol (CH₃ OH) solution containingcadmium chloride (CdCl₂) and then dried by virtue of natural drying tothereby form a CdCl₂ layer 41, as shown in FIG. 2F. In turn, the CdTelayer on which the CdCl₂ layer 41 is formed is annealed at 400° C. for15 minutes in an N₂ ambient or in the air. The solution used to dip maybe prepared by melting CdCl₂ of 1.1 g into a methanol of 1 liter whilestirring, and so on. The CdCl₂ film may be deposited or coated on theCdTe layer 4 by evaporation, sputtering, CVD, plating, spray method, orthe like, in place of the method dipping the substrate into the methanolsolution containing CdCl₂. In stead of the CdCl₂ film, chloride such asHCl, NH₄ Cl, etc. or chlorine (Cl₂) may be attached on the surface ofthe CdTe layer. Although the annealing temperature is desired to be setto 400° C., annealing may be carried out in the temperature range of 350to 600° C. This annealing is carried out to form the polycrystal CdTelayer with larger grain sizes, but the annealing temperature of 350° C.or less is not preferable because no enlargement of grain size iscaused. The annealing of 600° C. or more is not applicable since theoverall CdTe layer is sublimated if such annealing is carried out.

(6) In the end, after a predetermined pretreatment, e.g., slight etchingof a surface of the CdTe layer with the use of an etching solution suchas a K₂ Cr₂ O₇ +H₂ SO₄ +H₂ O solution, is carried out, as shown in FIG.2G, an ohmic electrode 5 is formed by forming a 10 nm thick Cu and a 100nm thick Au successively by the vacuum evaporation, and then annealingat 150° C. for about one hour in the N₂ +O₂ atmosphere or the air.Photovoltaic cells according to the first embodiment of the presentinvention can be then completed as shown in FIG. 3 if the resultantstructure is separated into photovoltaic cells of a predetermined sizesuch as 5 mm×5 mm by use of photolithography. FIG. 4 is a schematicsectional view showing the photovoltaic cells according to the firstembodiment of the present invention after having been finished.

                  TABLE 1    ______________________________________    Differences in cell characteristics    between the prior art and the first embodiment of the present invention             Voc(V) Isc(mA/cm.sup.2)                               FF(%)   Eff(%)    ______________________________________    Prior Art  0.62-0.71                        23-25      54-59  8.2-10.1    Present invention               0.78-0.82                        24-25      62-73 11.1-14.5    ______________________________________

Table 1 indicates comparison of various characteristics of thephotovoltaic cells manufactured according to the prior art and thepresent invention respectively. It would be appreciated that theopen-circuit voltage Voc and the fill factor FF can be improvedaccording to the present invention.

FIG. 5B is a view showing a spectral-response characteristic of thephotovoltaic cell according to the present invention, while comparingwith a spectral-response characteristic (FIG. 5A) of the photovoltaiccell in the prior art, when an applied voltage is varied. As shown inFIG. 5A, the spectral-response characteristic of the photovoltaic cellmanufactured in the prior art shows a fact that quantum efficiency issignificantly reduced when small forward voltage is applied. The factmeans that there exist a large number of defects to trap minoritycarriers when an electric field applied to the vicinity of the CdS/CdTejunction (in the depletion layer) is small. On the contrary, in thephotovoltaic cell manufactured by the manufacturing method according tothe first embodiment of the present invention, as shown in FIG. 5B,there is no reduction in quantum efficiency when the forward voltage isapplied. Thus it can be deduced that characteristics at the CdS/CdTeinterface and crystallographic quality are good enough.

It is of course that CdZnS, ZnS, etc. may be utilized as the n-typesemiconductor layer 3 and that CuInSe₂, CuInGaSeS, etc. may be utilizedas the p-type semiconductor layer 4. Also, the SnO₂ film or the ITO filmwhich is formed by oxidizing the tin film (Sn film) or the indium-tinalloy film (In-Sn alloy film) being formed by virtue of the evaporationmay be utilized in place of the In₂ O₃ film 20.

(Second Embodiment)

The feature of a second embodiment of the present invention lies in thestep of attaching the CdCl₂ onto the surface of CdTe layer. Namely, atabove-mentioned step (5) among the CdTe/CdS photovoltaic cellmanufacturing steps in the first embodiment, a substrate having the CdTelayer thereon is put into a sealed vessel, which shuts off the outsideair, after the methanol solution containing CdCl₂ is coated on thesurface of the CdTe layer, and then the substrate is warmed in thesealed vessel. If the methanol solution containing CdCl₂ is coated andthen dried by virtue of natural drying, like the first embodiment, it isdifficult to attach a large amount of CdCl₂ of more than 0.1 mg/cm² onthe surface of the CdTe layer. Furthermore, if such substrate is warmedmerely in the air, a drying time can be shortened but the inplaneuniformity is degraded. Therefore, in order to improve the inplaneuniformity in the second embodiment, the CdCl₂ can be attached to thesurface of the CdTe layer by putting the substrate into the vessel whichshuts off the outside air and then warming the vessel, as will bedescribed hereunder.

The manufacturing method according to the second embodiment of thepresent invention is in brief similar to the first embodiment. In otherwords,

(1) After the glass substrate such as the Corning 7059 substrate iscleaned, the ITO film or the SnO₂ film is formed 200 nm thick by thesputtering method, etc. (see FIG. 2B). The In₂ O₃ film is then formed bydepositing the In film 10 nm thick on this ITO film (or the SnO₂ film)by the vacuum evaporation and then annealing at 370° C. for 30 minutesin the air (see FIG. 2C). The SnO₂ film may be formed by depositing theSn film in place of the In film 10 nm thick on the ITO film by thevacuum evaporation. The n-type CdS layer is then formed 150 nm thick onthe In₂ O₃ film (or the SnO₂ film) at the substrate temperature of 350°C. by the vacuum evaporation (see FIG. 2D). In turn, the CdTe layer 4 isformed 4 μm thick at the substrate temperature of 350° C. by the vacuumevaporation, the sputtering method, the CVD method, or the like (seeFIG. 2E).

(2) Next, the substrate on which the p-type CdTe layer/the n-type CdSlayer are formed is put into the schale, then a surface of the resultantstructure is coated with the CdCl₂ contained methanol solution by use ofa pipette, and then the lid is put on the schale. The schale is thenplaced in an oven and then warmed at 80° C. for four minutes. A solutionwhich is coated by use of the pipette may be prepared by melting theCdCl₂ of 1.1 g into the methanol (CH₃ OH) of 1 liter, and so on.

(3) Succeeding steps are similar to those in the first embodiment. Inother words, if the ohmic electrode is formed by a predeterminedpre-treatment and the resultant structure is then separated into cellsof predetermined size, the CdTe/CdS photovoltaic cells according to thesecond embodiment of the present invention can be completed.

Characteristics of the photovoltaic cells according to the secondembodiment of the present invention and manufactured as above areindicated in a Table 2, while comparing with those of the photovoltaiccells in the prior art. Sample numbers 1, 2, and 5 provide thecharacteristics of the photovoltaic cells in the prior art whereassample numbers 3, 4, and 6 provide the characteristics of thephotovoltaic cells according to the second embodiment of the presentinvention.

                                      TABLE 2    __________________________________________________________________________    Difference in cell characteristics    between the prior art and the second embodiment of the present invention    Smpl.       Trans.             Oxide Film   Isc    No.       Cond. Film             Buffer  Voc(V)                          (mA/cm.sup.2)                               FF(%)                                    Eff(%)    __________________________________________________________________________    1  ITO   none    0.75-0.77                          23.5-23.8                               62.3-66.1                                    11.0-10.1    2  (˜10Q/Sq)             SnO.sub.2                     0.75-0.78                          23.9-24.7                               83.5-68.0                                    11.9-13.0             (sputter, 10 nm)    3        In evap.→oxid.                     0.82-0.84                          24.5-24.9                               71.4-78.6                                    14.5-16.0             (10 nm)    4        Sn evap.→oxid.                     0.83-0.84                          23.8-24.0                               72.1-73.9                                    14.4-14.7             (10 nm)    5  SnO.sub.2             none    0.71-0.81                          23.8-25.1                               63.8-70.1                                    11.5-12.9    6  (˜10Q/Sq)             In evap.→oxid.                     0.81-0.83                          23.2-24.3                               72.1-75.4                                    13.8-14.3             (10 nm)    __________________________________________________________________________

In case no oxide film buffer is provided (sample numbers 1 and 5), theconversion efficiency is at the lowest and the open-circuit voltage Voc,etc. are small. In case the SnO₂ film is formed directly on the ITO filmby the sputtering method (sample number 3), the conversion efficiency isin the range of 11.9 to 13.0% and the open-circuit voltage Voc is in therange of 0.75-0.78 V. In contrast, according to the second embodiment ofthe present invention, the conversion efficiency can be improved up to13.8 to 16.0% if the oxide film buffer is formed by depositing the Infilm by virtue of the vacuum evaporation and then oxidizing the In film(sample numbers 3, 6), or else by depositing the Sn film by virtue ofthe vacuum evaporation and then oxidizing the Sn film (sample number 4).In addition, it would be found that the open-circuit voltage Voc, theshort-circuit current Isc, and the fill factor FF can be improved ratherthan the prior art.

FIG. 6 is a view illustrating standard deviation in the conversionefficiency of the CdTe/CdS photovoltaic cell according to the secondembodiment of the present invention. In FIG. 6, standard deviation inthe conversion efficiency of the CdTe/CdS photovoltaic cell which iscoated with a large amount of the methanol solution containing CdCl₂ andthen dried by natural drying according to the first embodiment is alsoshown. It can be deduced from FIG. 6 that standard deviation in theconversion efficiency of the photovoltaic cell according to the secondembodiment of the present invention can be reduced in contrast to thefirst embodiment. In this case, the conversion efficiency of thephotovoltaic cell according to the first embodiment is an average of13.3% while the conversion efficiency of the sample shown in FIG. 6 isan average of 14.6%, which shows an improvement in the conversionefficiency.

Furthermore, it takes 30 minutes to dry the methanol solution in themethod employing the natural drying according to the first embodimentwhereas only four minutes are needed to dry the methanol solution in thesecond embodiment.

In the above description, though the case the annealing temperature isset to 80° C. has been explained, such temperature need not be limitedto 80° C. and can be selected appropriately in the range of 50° C. to100° C. A too much drying time will be required if the annealingtemperature is less than 50° C., while the annealing temperature inexcess of 100° C. is not preferable because the inplane uniformity isdegraded.

Still further, though the schale is exemplified as the sealed vesselwhich shuts off the outside air, another vessel which is equivalent instructure to the schale, a perfectly sealed receptacle, etc. may beemployed. It is preferable to pave the sealed vessel with hygroscopicmaterial such as silica gel. After a gas introducing port and a gasexhaust port are provided to the hermetically sealed vessel, thehermetically sealed vessel may be warmed while flowing dry air (N₂ +O₂),dry N₂ gas, high purity N₂ gas, or the like through the hermeticallysealed vessel.

Besides, chloride such as Cl₂, HCl, or NH₄ Cl other than CdCl₂ may beemployed.

As described earlier, according to the second embodiment of the presentinvention, a large amount of chloride such as CdCl₂ can be attached on asurface of the CdTe layer with good inplane uniformity in a short time,the conversion efficiency of the photovoltaic cell can be enhanced, andstandard deviation in the conversion efficiency can be made small.

As also described above, it would be evident that variouscharacteristics of the CdTe/CdS photovoltaic cell can be improvedaccording to the present invention.

In the second embodiment of the present invention, the n-typesemiconductor layer is not limited to CdS but CdZnS, ZnS, or the likemay be employed. Moreover, it is of course that the p-type semiconductorlayer 4 is not limited to CdTe but CuInSe₂, CuInGaSeS, or the like maybe employed. In place of the In₂ O₃ film, the SnO₂ film or the ITO filmmay be employed which can be formed respectively by forming the Sn filmor the In-Sn alloy film by virtue of vacuum evaporation and thenoxidizing the Sn film or the In-Sn alloy film.

Like the above, although the present invention has been set forth withreference to the first and second embodiments, it should not beunderstood that the present invention is limited by such descriptionsand drawings constituting a part of this disclosure. From thisdisclosure, it is evident for one skilled in the art that variousalternative embodiments and application techniques may be adopted. Inthis manner, it would be understood that the present invention maycontain various embodiment not referred to herein. Accordingly, thepresent invention is to be defined only by the configuration set forthin the appropriate appended claims based on this disclosure.

INDUSTRIAL APPLICABILITY

As has been explained above, according to the present invention, thestrain caused when the n-type and p-type semiconductor layers,constituting the photovoltaic cell, are grown in grain size can besuppressed by the presence of the indium oxide film (In₂ O₃ film), thetin oxide film (SnO₂ film), or the indium-tin oxide film (ITO film)serving as a strain relaxation layer. Hence, good pn heterojunction suchas CdS/CdTe junction can also be obtained, and performances of thephotoelectric conversion device can be extremely improved.

In particular, by warming/drying the CdCl₂ contained methanol solutionin the sealed vessel, a large amount of CdCl₂ can be formed uniformly onthe p-type semiconductor layer and also the photoelectric conversiondevice with high conversion efficiency can be accomplished.

The present invention can make a beginning of the solution of energyproblems that human beings are now confronted with. In other words, thepresent invention has important applicability in the energy industrialfields as alternative energy technology other than the hydrocarbon-basedresource. For instance, the present invention can become thecommencement to the technology which make possible manufacture of thephotovoltaic cell to generate remote or relatively large scaledistribution electric power. According to the present invention, it isfeasible to manufacture the suitably efficient photovoltaic cell at lowcost. Although a single crystal photovoltaic cell is employed inrelatively greater part of the photoelectric conversion device of today,it has in essence a high ratio of cost/output watt. According to thepresent invention, the highly efficient polycrystal photovoltaic cellcan be achieved at low cost. In other words, according to the presentinvention, material and manufacturing cost can be readily cut down.Therefore, the high efficiency and low cost polycrystal photovoltaiccell according to the present invention is also applicable to suchindustrial fields that need relatively low cost electrical power supplymeans at remote locations, such as telecommunication stations,agricultural water pumping sites, remote villages, and portable housingfacilities. The technology according to the present invention, as anatural consequence, can discover the applicability in the technicalfields such as future photovoltaic power plants which compete withconventional hydrocarbon consuming plants.

We claim:
 1. A method of manufacturing a photoelectric conversion devicecomprising at least the steps of:(a) a first step of forming an indiumoxide film, a tin oxide film, or an indium-tin oxide film by forming anindium film, a tin film, or an indium-tin alloy film on a transparentconductive substrate and then annealing at a temperature of more than300° C. in oxygen containing atmosphere; (b) a second step of forming ann-type semiconductor layer on an indium oxide film, a tin oxide film, oran indium-tin alloy oxide film and then forming a p-type semiconductorlayer on the n-type semiconductor layer; and (c) a third step ofenlarging grain size of polycrystals of the p-type semiconductor layer.2. The method of manufacturing a photoelectric conversion deviceaccording to claim 1, wherein a thickness of the indium film, the tinfilm, or the indium-tin alloy film is selected in a range of 2.5 nm to100 nm.
 3. The method of manufacturing a photoelectric conversion deviceaccording to claim 2, wherein the indium film, the tin film or theindium-tin alloy film is formed by a vacuum evaporation method.
 4. Themethod of manufacturing a photoelectric conversion device according toclaim 2, wherein the n-type semiconductor layer is formed of an n-typeCdS layer, the p-type semiconductor layer is formed of a p-type CdTelayer, the third step is made up of steps of forming a chloride layer onthe p-type CdTe layer and then annealing.
 5. The method of manufacturinga photoelectric conversion device according to claim 4, wherein thechloride layer formed in the third step is formed of a cadmium chloridelayer.
 6. The method of manufacturing a photoelectric conversion deviceaccording to claim 2, wherein the transparent conductive substrate isformed of a predetermined transparent substrate on which a transparentconductive film is formed.
 7. The method of manufacturing aphotoelectric conversion device according to claim 2, wherein thethickness of the indium film, the tin film, or the indium-tin alloy filmis selected in a range of 2.5 nm to 50 nm.
 8. The method ofmanufacturing a photoelectric conversion device according to claim 7,wherein the indium film, the tin film or the indium-tin alloy film isformed by a vacuum evaporation method.
 9. The method of manufacturing aphotoelectric conversion device according to claim 7, wherein the n-typesemiconductor layer is formed of an n-type CdS layer, the p-typesemiconductor layer is formed of a p-type CdTe layer, the third step ismade up of steps of forming a chloride layer on the p-type CdTe layerand then annealing.
 10. The method of manufacturing a photoelectricconversion device according to claim 9, wherein the chloride layerformed in the third step is formed of a cadmium chloride layer.
 11. Themethod of manufacturing a photoelectric conversion device according toclaim 7, wherein the transparent conductive substrate is formed of apredetermined transparent substrate on which a transparent conductivefilm is formed.
 12. The method of manufacturing a photoelectricconversion device according to claim 1, wherein the indium film, the tinfilm or the indium-tin alloy film is formed by a vacuum evaporationmethod.
 13. The method of manufacturing a photoelectric conversiondevice according to claim 1, wherein the n-type semiconductor layer isformed of an n-type CdS layer, the p-type semiconductor layer is formedof a p-type CdTe layer, the third step is made up of steps of forming achloride layer on the p-type CdTe layer and then annealing.
 14. Themethod of manufacturing a photoelectric conversion device according toclaim 13, wherein the chloride layer formed in the third step is formedof a cadmium chloride layer.
 15. The method of manufacturing aphotoelectric conversion device according to claim 1, wherein thetransparent conductive substrate is formed of a predeterminedtransparent substrate on which a transparent conductive film is formed.